The present invention relates generally to projection television sets, particularly to projection television sets having improved optical characteristics, and more particularly to a protection panel for a rear projection television set that has a spectral transmittance that enhances the quality of a color image projected through the protection panel.
Projection television sets, particularly rear projection television sets, are a popular alternative to picture tube television sets, as they provide relatively large viewable screens compared to conventional picture tubes. As shown in FIG. 1, a rear projection television set 10 generally includes a cabinet 12 with a set of internal projection tubes 14, a mirror 16, and electronic circuitry (not shown) for receiving broadcast signals and the like, and/or for controlling the projection of an image onto a screen assembly 18 mounted to the front of the cabinet 12.
Turning to FIG. 2, each projection tube 14 includes a cathode ray tube (xe2x80x9cCRTxe2x80x9d) 20, a spacer 22 attached to the CRT 20, and a lens assembly 24 mounted to the spacer 22. The lens assembly 24 includes one or more individual lenses, such as a condenser lens 26 and a xe2x80x9cC-lensxe2x80x9d 28, mounted in a lens barrel 30, generally with a cooling fluid 34 provided between the CRT 20 and the C-lens 28, as is known in the art. The CRT 20 receives signals from the electronic circuitry that excite a fluorescent substance 32 in the CRT 20 such that the projection tube 14 emits an image (not shown), generally of a single color of light, towards the mirror 16. Generally, a set of three projection tubes 14 are provided (only one shown), each projection tube 14 emitting an image in one of the primary colors, i.e., substantially red, green, and blue light, respectively.
Returning to FIG. 1, the mirror 16 reflects and/or focuses the images (represented generally by ray of light 36) from the projection tubes 14 towards the screen assembly 18. The screen assembly 18 may include a fresnel lens 38 that may further condense the images emitted by the projection tubes 14, and a lenticular screen 40 that may correct and/or control the directional projection of the images, as is known in the art. Finally, an outer protection panel 42 is provided that protects the internal components, such as the lenticular screen 40, and allows the images from the projection tubes 14 to be seen from the front of the cabinet 12.
The fluorescent substances provided in each of the CRT""s of a conventional set of projection tubes generate emission spectra that are well known. FIG. 3 shows the relative intensity or luminance of light emitted by a set of commonly used CRT""s as a function of the wavelength of the light. For example, a blue CRT generally emits light that corresponds to curve 51, having a peak around a dominant wavelength of about 450 nm. A green CRT generally emits light that corresponds to curve 52, having a dominant wavelength of about 545 nm, and including several sidebands 52A-52D. Finally, a red CRT generally emits light that corresponds to curve 53, having a dominant wavelength of about 610 nm, and including sidebands 53A-53B.
The sidebands 52A-52D, 53A-53B are undesirable emissions generated by the fluorescent materials, and generally reduce the overall color purity of the resulting images that are projected onto the screen assembly 18. To correct for these sidebands, it has been suggested to provide filters in the red and green projection tubes. For example, a color absorbing material may be provided in one of the lenses of the projection tubes, such as the C-lens 28, as is disclosed in U.S. Pat. No. 5,010,396 issued to Hanyu et al. Alternatively, a color absorbing material may be included in the cooling fluid 34 within the projection tube 14, as is disclosed in U.S. Pat. No. 5,055 922 issued to Wessling.
For example, a colored C-lens for a red CRT may provide a filter characteristic such as that shown FIG. 4. The xe2x80x9cspectral transmittancexe2x80x9d (i.e., the ratio of the intensity of the light exiting from a material as compared to the intensity of the incident light entering the material, as a function of wavelength of the light) for this colored C-lens may provide a transmittance above about 600 nm that is substantially greater than ninety percent (90%),e.g., about ninety two percent (92%). Below 600 nm, however, the transmittance decreases steeply, thereby substantially removing, for example, light from the sideband 53A (shown in FIG. 3), and consequently improving the purity of the red image.
In addition to problems of color purity, rear projection televisions may also experience problems with contrast due to light sources external to the television set. xe2x80x9cContrastxe2x80x9d is defined as a ratio of the intensity of light that is received by an observer from a screen displaying one hundred percent (100%) white, and the intensity of light that is received from a screen displaying zero percent (0%) black. If undesired light is seen when the projection television is displaying zero percent (0%) black, it may appear to be brighter than normal black, resulting in a contrast that is lowered and a picture quality that appears to be deteriorated.
For example, as shown in FIG. 5, light from a light source 44 may interfere with the images projected through the protection panel 42, such as from a lamp in the room where the projection television set is located or sunlight from a nearby window. The light source 44 may emit rays of light 46, some of which may be directed towards the screen assembly 18, and be at least partially reflected back towards an observer 48 trying to watch the projection television 10.
An exemplary ray of light 46A from light source 44 is shown that may strike an outside surface 42A of the protection panel 42. As the ray 46A enters and passes through the protection panel 42, first and second reflected rays 46C, 46E may be reflected off of the outside and inside surfaces 42A, 42B of the protection panel 42. In addition, a portion of the ray 46F may leave the protection panel 42 and strike the outside surface 40A of the lenticular screen 40. Although the ray 46F may be diffused by the outside surface 40A of the lenticular screen 40, it may generate a third reflected ray 46G, a portion of which may exit the protection panel 42 as ray 46I and be directed towards the observer 48.
All of these reflected rays 42C, 42E, 42I may interfere substantially with the images being projected by the projection television 10, represented by image ray 36 that passes through the screen assembly 18 towards the observer 48 as ray 36E. For example, a conventional protection panel may reflect about four percent (4%) of incident light striking its surface. Thus, a substantial amount of undesired light (as much as eight percent (8%) or more of the incident light) from a light source 44 may be reflected from the screen assembly towards an observer 48, thereby substantially reducing the contrast of the images being viewed on the protection panel 42.
To reduce the effects of this undesired external light, it has been suggested to add color absorbing material to the protection panel, resulting in what is known as a xe2x80x9cdark tintxe2x80x9d protection panel. Such dark tint protection panels generally have a spectral transmittance that is reduced by about twenty percent (20%) as compared to non-tinted protection panels. The spectral transmittance is substantially uniform across all wavelengths, such that the protection panel does not substantially affect the color of the images projected through it. FIG. 6 comparatively illustrates an exemplary spectral transmittance of a nontinted protection panel (line 61) and a dark tint protection panel (line 62). The spectral transmittances shown include a four percent (4%) reduction in intensity of the light passing through the respective panels due to reflection from each of the outside and inside surfaces, as discussed above.
Because of the tinting, any rays that pass through the dark tint protection panel are substantially reduced in intensity. For example, referring again to FIG. 5, because reflected rays 46E and 46I pass through the protection panel twice before being experienced by the observer 48, they may have a relative intensity that is sixty four percent (64%) (0.80xc3x970.80) that of similar rays passing through a non-tinted protection panel.
Stated differently, the rays have an intensity reduction by twenty percent (20%) each time they pass through the dark tint protection panel before reaching the observer 48.
Dark tint protection panels, however, also reduce the intensity of the light 36 emitted by the projection tubes (e.g., by twenty percent (20%)), and therefore reduce the intensity of the images visible through the protection panel 42. For example, because the ray 36E that reaches the observer 48 passes through the protection panel, it also experiences a twenty percent (20%) reduction in intensity. Because the external light rays 46E, 46I pass through the protection panel 42 twice, however, there is still a net gain in contrast, as compared to a non-tinted protection panel.
To compensate for the loss in intensity of the resulting images projected through a dark tint protection panel (and also due to the loss through the lenticular screen 40, which generally has a spectral transmittance of about seventy percent (70%)); the intensity of the light being emitted by the projection tubes may be increased, e.g., by simply increasing the current to the CRT""s. This solution, however, may cause xe2x80x9csaturationxe2x80x9d of the fluorescent material in one or more of the CRT""s, such that any further increase in current does not result in a proportional increase in intensity of the light emitted by the respective saturated CRT. Blue CRT""s, in particular, may be susceptible to color saturation, such that increasing the current to the projection tubes too high may result in a loss of blue color in the resulting images, and thereby reducing the overall color quality of the projection television.
Accordingly, it would be desirable to provide a projection television set that has improved color purity and/or contrast.
The present invention is directed to protection panels for rear projection television sets that enhance the color quality and/or contrast of the television sets. In accordance with one aspect of the present invention, a protection panel is provided that includes a substantially planar member having a spectral transmittance that peaks at dominant wavelengths of substantially red, green, and blue light, and that is reduced substantially at wavelengths between red and green light and between green and blue light. Preferably, the spectral transmittance at a wavelength between red and green light is at least about five percent less than at the dominant wavelengths of substantially red and green light, and/or the spectral transmittance at a wavelength between green and blue light is at least about five percent less than at the dominant wavelengths of substantially green and blue light. More preferably, the spectral transmittance at a dominant wavelength of substantially red light is at least about five percent less than at a dominant wavelength of substantially blue light.
In addition, the protection panel may include an anti-reflection film on one or both surfaces of the planar panel. A frame may be provided that extends around a perimeter of the planar panel. Further, the protection panel may include a connector for detachably mounting the protection panel to a rear projection television set.
In accordance with another aspect of the present invention, a rear projection television set is provided that includes a cabinet having an opening in a front panel thereof communicating with a cavity therein, and a set of projection tubes in the cavity for projecting images towards the opening, each projection tube configured for emitting substantially single color images in one of red, green, and blue light. A screen assembly is mounted across the opening for viewing the images from outside the cabinet. The screen assembly includes a protection panel having a spectral transmittance that peaks at wavelengths of substantially red, green, and blue light, and that is reduced substantially at wavelengths between red and green light and between green and blue light, such as the protection panel described above. A mirror may also be mounted in the cavity in a predetermined relationship with the set of projection tubes such that the images projected from the set of projection tubes are reflected towards the screen assembly.
In addition, the screen assembly may also include a lenticular screen adjacent the protection panel, for example, having a spectral transmittance of at least about eighty percent.
The screen assembly may also include a fresnel lens adjacent the protection panel.
The projection tubes each include a CRT for emitting substantially single color light, and may include a filter, for example, one of the lenses therein may include a color absorbing substance, for filtering one or more undesired sidebands generated by the CRT. For example, a red projection tube may include a lens for filtering the substantially red light emitted by CRT, the lens having a spectral transmittance that substantially absorbs all light below a wavelength of about 600 nm and absorbs at least about ten percent of light at or above a wavelength of about 600 nm.
In a preferred embodiment of the projection television set, the protection screen may be removable from the cabinet. The projection television set may include a user interface for selecting one of an unshielded and a shielded mode indicating whether the protection panel is removed or mounted to the cabinet, respectively. Control circuitry may be coupled to the interface for adjusting current levels of the set of projection tubes in response to the selected mode to provide a predetermined color temperature of an image visible from outside the cabinet through the screen assembly. In one form, where the protection panel has a spectral transmittance at a dominant wavelength of substantially red light that is at least about five percent less than at a dominant wavelength of substantially blue light, the control circuitry may be configured for increasing a color temperature of the set of projection panels when the shielded mode is selected, thereby automatically providing a desired effective color temperature for the images visible through the screen assembly of the projection television set.
In accordance with yet another aspect of the present invention, a rear projection television set is provided that includes a cabinet having an opening in a front panel thereof communicating with a cavity therein, and a set of red, green, and blue projection tubes in the cavity for projecting images towards the opening. Each projection tube is configured for emitting substantially single color images in one of red, green, and blue light, respectively, the red projection tube including a lens having a spectral transmittance of ninety percent or less.
A screen assembly is mounted across the opening for viewing the images from outside the cabinet, the screen assembly including a lenticular screen having a spectral transmittance greater than about seventy percent, and a protection panel having a spectral transmittance that peaks at wavelengths of substantially red, green, and blue light, and that is reduced substantially at wavelengths between red and green light and between green blue light.
Because the protection panel has a relatively high transmittance around the red, green, and blue wavelengths, while attenuating the light intermediate to the dominant wavelengths, the protection panel may substantially enhance the color quality of images observed on the projection television set. In addition, where the protection panel has a transmittance that is greater for blue light than for red light, the protection panel may also reduce the risk of color saturation of the blue projection tube, thereby increasing the color purity at increased brightness levels.